The present invention relates to a magnetooptic read head.
To read a magnetic medium, a known type of magnetooptic head uses a Faraday effect and/or a Kerr effect. These effects consist in magnetizing a magnetooptic material, such as garnet, by the magnetic flux produced by the magnetic medium, and in detecting this magnetization by means of the rotational power it has on polarized light.
The physical principle exploited is the magnetooptic effect known by the name of Kerr rotation or Faraday rotation. In this effect, the polarization state of an electromagnetic wave reflected from a magnetooptic medium or passing through it, is modified. This state depends on the direction of magnetization of the medium. Since the read pole consists of a material of this type, the polarization of a laser beam reflected therefrom is altered by the variations of magnetization induced by the magnetic medium. This modulation is then transformed into variations of light intensity which are then measured using a photodetector thus making it possible to reconstruct the information written on the magnetic medium. Therefore the magnetic information is no longer carried by an electric current but by an electromagnetic wave.
A read head of this type has been described in French patent No. 2 656 732. FIG. 1 shows the simplified diagram of one embodiment of this head.
It comprises a layer 9 of magnetic material with good magnetic permeability, of a thickness which can reach several microns, for example.
A layer 10 of nonmagnetic material, of a thickness for example between 50 and 3 000 nm, and typically about 300 nm, and a thin layer 11 of magnetic material forming the layer with a magnetooptic effect (Kerr effect for example) are formed on the layer 9. The thickness of the layer 11 is, for example, between about 10 and 200 nm. An oblique incident optical beam 12 arrives on the layer 11, which reflects a beam 13. The layers 9 to 11 are polished on one of their sides, for example perpendicularly to their main surface, and the magnetic tape 14 to be read is applied to the surface polished in this way.
The head of FIG. 1 may be easily produced in a single layer deposition operation, and it is not absolutely necessary to etch the deposited layers in order to produce multitrack read heads.
FIG. 2 shows a multitrack magnetooptic read system 20 comprising a read head according to the invention. The main benefit of using a Kerr effect head for reading a multitrack recording is due to the principle of active reading, the performance of which is not disadvantaged by a low play speed of the tape to be read. Upstream of the Kerr effect head 21, the system 20 comprises a point light source 22, preferably a laser diode, a collimating objective 23, a device 24 comprising a polarizer serving to make the polarization of the light beam passing through it perpendicular to the direction of movement of the tape 25 to be read, and a cylindrical lens 26 oriented so as to focus the beam collimated by the objective 23 along a line perpendicular to the direction of movement of the tape 25.
The head 21 comprises, as stated in the present description, a Kerr effect sensor and, as necessary, an optical reflector.
Downstream of the head 21, the system 20 comprises an optical imaging device 27; a device 28 comprising an analyzer and, where necessary, a phase compensator; and a linear optical sensor 29, the active zone of which is the optical conjugate, by means of the optical system, of the line illuminated on the Kerr effect head. This optical sensor comprises, for example, a CCD linear array. Since the magnification of the downstream part of the system 20 is typically about one, it is possible advantageously to produce it in integrated optics.
FIG. 3 shows one embodiment of a magnetooptic read head according to the invention in which the various layers of FIG. 1 are produced on a prism 35 made of a material transparent to the wavelength of the read beam. This material is, for example, made of GGG (galidonium gallium garnet). The various faces 30 to 33 of this prism are oriented such that the read beam 12 penetrates without reflection into the prism and is reflected on the face 32 bearing the magnetooptic layer 11 of the head, near to the read zone of the magnetic tape 14 to be read. The exit face 33 is oriented such that the beam 13 exits without reflection from the prism. It should be noted that the faces 30 and 33 may be coated with a layer of antireflective material made of SiO2 for example.
The layers 9, 10, 11 correspond to the layers bearing the same references in FIG. 1 are made on the face 32 of the prism.
In this type of known magnetic head, the magnetooptic layer 11 is made of sendust (FexSiyAlz), the airgap layer 10 is made of Si3N4 or Al2O3 and the magnetic layer 9 is made of sendust.
By way of example, at the present time, a magnetooptic read head may consist of a stack of the following materials:
a sendust layer 11, with a thickness of 30 nm and which acts as a read pole,
a silicon nitride layer with a thickness of 180 nm,
a copper layer with a thickness of 50 nm
these two layers acting as a magnetic and optical airgap layer 10.
a sendust layer 9 with a thickness of 1 xcexcm making it possible to close the flux and acting as magnetic shielding,
a layer of alumina or of Si3N4 or of any other hard nonmagnetic material with a thickness of 2 xcexcm can be provided to protect the assembly.
All of this is deposited on a garnet substrate.
These materials of a very different nature all have different mechanical properties. Thus, we have a combination of hard materials (garnet, Si3N4, Al2O3) and of soft materials (sendust, copper). This construction of materials with very different hardnesses is extremely penalizing in terms of component service life. This is because, on contact with the tape, we observe a phenomenon of differential wear between the hard and soft materials with the appearance of a recess of about 100 nm in the less hard materials, this recess being mainly located on the layer 9 (see FIG. 4a). This wear is shown in FIGS. 4a and 4b. Where the layer 9 is covered with a layer 10b of hardness greater than that of the layer 9, wear of the sort shown in FIG. 4b may occur. This wear may be substantial at the end of only a few hundred hours of use, while it is desired to have from 5 000 to 30 000 hours of operation depending on the intended application. This recess results in a signal loss of about 6 dB at 1 xcexcm of electromagnetic wavelength due to the space created and the loss of resolution, the head passing progressively from bipolar operation where the resolution is determined by the gap width to monopolar operation where the resolution is then determined by the extent of the active zone of the read pole.
It is then impossible to recover the signal, the part in contact being irreparably damaged and only reforming the flat would allow this signal to be recovered.
In magnetoresistive read/write heads, the current solutions propose depositing a layer of a hard material on the front of the head. However, this solution is demanding and expensive in terms of technical production (the hard materials being difficult to produce), and generates space losses of about 3 to 4 dB at 1 xcexcm (for 60 nm deposited).
The invention aims to solve this wear problem.
The invention therefore relates to a magnetooptic read head, characterized by the fact that it comprises a magnetooptic transducer with a multilayer structure with at least one thin magnetic layer with a magnetooptic effect, at least one layer of a nonmagnetic material and having a predetermined wear coefficient and a layer with good magnetic permeability for closing a magnetic circuit, and in that the layer with good magnetic permeability comprises alternating first sublayers made of a magnetic material with good magnetic permeability and second sublayers made of a material having a wear coefficient substantially equivalent to said wear coefficient of the layer made of a nonmagnetic material.